Automatic rotamoulding apparatus and method of control

ABSTRACT

A method for controlling an automatic rotamolding process, the method including the steps of: metering a first prescribed quantity of plastics material powder into a hopper to be delivered to a rotatable mold as a first shot; heating the mold; when the mold reaches a prescribed temperature, delivering the first shot of powder from the hopper to the mold and simultaneously rotating the mold so as to distribute the powder over an interior surface of the mold; heating the mold in accordance with a predetermined temperature-time characteristic; cooling the mold; and ejecting the finished rotamolded product from the mold.

FIELD OF THE INVENTION

The present invention relates to rotational moulding apparatus andprocesses and relates particularly, though not exclusively, to anautomatic rotamoulding apparatus and method of control.

BACKGROUND TO THE INVENTION

Rotational moulding (or rotamoulding) is a process for manufacturinghollow plastics products in which a powdered thermoplastic material,typically polyethylene and/or polypropylene, is melted in a heated mouldand in which there is typically no pressure applied to cause theplastics material to take up the shape of the mould. Plastics powder isplaced in one half of a mould which is then closed and rotated in anoven. Typically the mould itself is rotated biaxially about twoperpendicular axes, although in some rotamoulding systems the mould isrotated about one axis inside an oven, and the oven itself and/or themould is pivoted about a second perpendicular axis (the so-called "rockand roll" rotamoulding system).

As the mould rotates in the oven it heats up, typically to around 200°C., and the plastics material powder starts to melt and coat the innersurface of the mould. Due to the biaxial movement of the mould thepowder distributes itself substantially uniformly over the entire innersurface of the mould. When all the powder is melted, the mould is cooledto cause the melted plastics material to solidify in the desired shape.Rotation of the mould is then ceased and it can then be opened to removethe plastic product. The advantages of rotamoulding are increasinglybeing recognised by manufacturers, including the relatively low cost ofmoulds, the production of stress-free products (always a problem withinjection moulding) and the possibility of novel shapes and designs.

Current rotamoulding machines and processes are relativelylabour-intensive. Manual intervention is typically required to open andclose the mould and to place the powder in the mould. Moving the mouldinto and out of the oven and into a cooling station may also beperformed manually. Furthermore, due to the biaxial rotation of themould typically very little control can be exercised over the processwithin the mould and therefore there is a heavy reliance on experienceand trial and error procedures. This latter problem of process controlhas been overcome to some extent with the development of the rotationalmoulding process control system developed by The Queens University ofBelfast and described in WO 91/0564 ("the ROTOLOG system").

The ROTOLOG system monitors the air temperature inside the mould andtransmits this information to a remotely located computer. The real timeoutput from the ROTOLOG system allows the machine operators and qualitycontrol personnel to identify the time at which all the powder ismelted, the maximum internal air temperature within the mould, the timeat which the product has solidified and the heating/cooling rates duringthe rotamoulding process. Whilst the ROTOLOG system has removed much ofthe guesswork from the rotamoulding process, it has not reduced thelevel of manual intervention required. Furthermore, because the ROTOLOGsystem measures the air temperature within the mould, rather than theactual melt temperature or mould temperature it cannot identifytemperature variations on the interior surface of the mould which mayhave a significant impact on the characteristics of the finishedproduct.

SUMMARY OF THE INVENTION

The present invention was developed with a view to providing arotamoulding apparatus and method of process control in which arotamoulded product can be produced with minimal or no manualintervention.

According to one aspect of the present invention there is provided anautomatic rotamoulding apparatus comprising:

a rotatable mould mounted for rotation, having heating means provided inconnection therewith for heating the mould directly during rotation andsensing means provided in connection therewith for sensing thetemperature of the mould;

delivery means for automatically delivering a plastics material powderto the mould during rotation; and,

electronic control means operatively connected to said delivery means,said heating means and said sensing means, for controlling the deliveryof plastics material to the mould and for controlling the temperature ofthe mould throughout the rotamoulding process responsive to said sensingmeans whereby, in use, a rotamoulded product can be produced in themould without manual intervention.

In a preferred embodiment, said rotatable mould is one of a plurality ofrotatable moulds mounted in a plurality of modules, each module beingindependently operable under the control of said electronic controlmeans. Typically each mould comprises first and second componentsmovably mounted within a first support frame adapted for rotation abouta first axis, said first component of the mould being movable relativeto the second component between a first position in which the mould isclosed and ready to receive a shot of plastics material powder, and asecond position in which the mould is opened and the finishedrotamoulded product can be removed. In the preferred embodiment eachmould further comprises actuating means operatively connected to saidelectronic control means for moving the first and second componentsbetween said first and second positions.

Advantageously said heating means comprises a plurality of electricheating elements provided in thermal contact with the first and secondcomponents of the mould and said sensing means comprises a plurality oftemperature sensing devices provided in connection with the first andsecond halves of the mould in predetermined locations.

Preferably each rotatable mould further comprises cooling means tofacilitate forced cooling of the mould following heating. Typically saidcooling means comprises a passage extending proximate to said first andsecond components of the mould and pumping means operatively connectedto said electronic control means for pumping cooling medium through saidpassages.

Preferably the automatic rotamoulding apparatus further comprisesconveyor means for conveying a finished rotamoulded product from themould to a location remote from the mould. In one embodiment theapparatus is also provided with stacking means for stacking the finishedrotamoulded products as they arrive at said remote location.

According to another aspect of the present invention there is provided amethod for controlling an automatic rotamoulding process using anelectronic control means operatively connected to a delivery means, aheating means and a sensing means, the method comprising the steps of:

metering a first prescribed quantity of plastics material powder into ahopper, to be delivered to a rotatable mould as a first shot;

operating said heating means provided in connection with the mould toheat the mould;

actuating said delivering means to deliver the first shot of powder fromsaid hopper to the mould and simultaneously rotating the mould so as todistribute the powder over an interior surface of the mould;

monitoring said sensing means provided in connection with the mould tosense the temperature of the mould during rotation;

controlling the operation of said heating means in accordance with apredetermined temperature-time characteristic and responsive to saidmonitoring of the sensing means;

cooling the mould; and,

ejecting the finished rotamoulded product from the mould.

Preferably the method further comprises the steps of:

conveying the finished rotamoulded product on a conveyor to a stackingapparatus; and,

stacking the finished rotamoulded product to form a stack of suchfinished rotamoulded products.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

In order to facilitate a better understanding of the nature of theinvention a preferred embodiment of the automatic rotamoulding apparatusand method of control will now be described in detail, by way of exampleonly, with reference to the accompanying drawings, in which:

FIG. 1 is a side elevation of a preferred embodiment of the automaticrotamoulding apparatus according to the invention, in the form of anAutomatic Pallet-making Machine (APM);

FIG. 2 is a plan view of the APM of FIG. 1;

FIG. 3 illustrates schematically part of the APM of FIG. 1;

FIG. 4 is a plan view of a preferred embodiment of a rotatable mouldassembly for the APM of FIG. 1, mounted in a pair of biaxially rotatableframes;

FIG. 5 is a side view of the rotatable mould assembly of FIG. 4;

FIG. 6 illustrates a finished pallet being ejected onto a conveyor meansfor conveying the pallet to a preferred stacking means;

FIG. 7 is an end elevation of the stacking means of FIG. 6;

FIGS. 8(a) and 8(b) are a schematic block diagram illustrating thepneumatic circuit, flow path of powder, and the electric power andcontrol circuits in a control module of the APM of FIG. 1;

FIGS. 9(a) 9(b), 9(c) and 9(d) are schematic block diagrams illustratingthe pneumatic circuit, the flow path of powder and the electric powerand control circuits in a production module of the APM of FIG. 1;

FIGS. 10(a) 10(b) are a flow diagram illustrating the control sequenceof the APM of FIG. 1; and,

FIG. 11 is a graphical representation of a typical temperature timecharacteristic of a heating cycle for the mould of FIG. 4.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

The following detailed description will be given with respect to anAutomatic Pallet-making Machine (APM) for mass-producing plastic palletsusing a rotamoulding process. A huge number of pallets are manufacturedand used world-wide for transporting and storing loads. At present, themajority of such pallets are still manufactured from wood materials,although an increasing number are being manufactured from plasticsmaterials. In order to compete with wooden pallets, plastic pallets mustbe of equal strength and their manufacture must be economicallyfeasible. The rotamoulding process is well-suited to this application,since the pallets can be made of hollow construction and with carefuldesign can have a mechanical strength and handleability which isequivalent, if not better, than wooden pallets.

It will be readily understood that the automatic rotamoulding apparatusand method of control described can also be used to manufacture otherrotamoulded products, and is not limited to manufacturing plasticpallets. With minor modifications and the appropriate moulds, theautomatic rotamoulding apparatus herein described can be used tomanufacture any product which is amenable to rotational moulding.

FIGS. 1 and 2 illustrate an automatic rotamoulding apparatus in the formof an Automatic Pallet-making Machine (APM) 10 which comprises fourmodules, a control module 12 and three substantially identicalproduction modules 14 which in this embodiment are arranged in a lineararray to form a pallet production line. A catwalk 16 provided above theAPM 10 allows a process operator and/or observers to walk along thelength of the machine to observe the operation of each of the modules12, 14 from above. Side walls 18 of the modules 12, 14 are mounted onhinges and can be swung open to allow access for maintenance purposes orto modify the configuration of the APM 10. A conveyor 20 extends thefull length of the three production modules 14 and is adapted to carrythe finished pallets to a stacking means in the form of a pallet stacker22.

Control module 12 controls the operation of the production modules 14and includes electronic control means comprising one or more computerterminals 24 mounted in a control console in the side wall 18 of thecontrol module 12. The electronic control means also comprisesassociated peripheral devices and remote terminal units (RTUs) orprogrammable logic controllers (PLCs). The video display units (VDUs) ofthe computer terminals 12 provide visual information and feedback to themachine operator of numerous control parameters and the operation of theAPM 10. The control console of control module 12 may also includevarious other visual and aural indicators, for example, an LED displayto monitor the operation of the APM 10. The keyboards of computerterminals 24 allow the operator to program the APM with a required"recipe" for the rotamoulding process and to access various levels ofthe process control program. The electronic control means may comprise aremote computer interface which communicates with one or more PLC's onthe production module(s).

The process control program typically provides several security levelsof access, so that the owner of the APM can prevent unauthorised accessto sensitive proprietary information concerning the control process andproduct "recipe". Thus, for example, only the first two levels can beaccessed by the operator, whereas the owner of the APM 10 can accessother levels including various reporting functions concerning themachine operation. In the case of a leased machine, the supplier of theAPM 10 may access still further levels, including diagnostics and otherreporting systems to control the licensed use of the machine. Theelectronic control means may also include a modem to allow on-linecommunication via the public telephone network between the supplier andthe APM 10. In this way, the supplier can interrogate the electroniccontrol means of the APM for fault-finding purposes or to obtain anupdate from various reporting systems. The operation of the electroniccontrol means and control software will be described in more detailbelow.

The control module 12 typically houses two primary hoppers 26 forcontaining for example, two different types of plastic material powderrespectively. Each primary hopper 26 is provided with a metering means28, (see FIG. 3) for metering a predetermined quantity of plasticsmaterial powder into respective secondary hoppers 30. Metering means 28may be a metering screw or butterfly valve. The secondary hopper 30 isprovided with an exhaust valve 35 to exhaust air back into the primaryhopper 26 when the secondary hopper is filling with powder. Eachsecondary hopper 30 is fitted with a fluidiser tube 32 which fluidisesthe granules of powder in an air stream for delivery via transfer lines34 to a diverter valve 36 (not illustrated in FIG. 3). The divertervalve 36 is connected to a powder transfer line 38 through which theplastics material powder is transferred to a rotatable mould 40 in oneof the production modules 14. The primary and secondary hoppers 26, 30,metering means 28, transfer lines 34, 38, fluidiser tubes 32 and thediverter valve all constitute part of a preferred form of delivery meansfor delivering plastics material powder to the rotating mould 40 duringrotation. This preferred form of delivery means in which the powder iscarried in an air stream to the rotating mould 40, is fully described inthe specification which accompanied Australian Provisional ApplicationNo. PN0666 the contents of which are incorporated herein by reference.

FIGS. 8 and 9 are schematic block diagrams illustrating the pneumaticcircuit and flow path of powder in the APM 10, as well as the electricpower and control circuits in the APM 10. FIG. 8 illustrates those partsof the respective circuits found in the control module 12, whilst FIGS.9(a) and 9(b) illustrate those parts of the circuits found in theproduction modules 14.

Referring to FIG. 8, it can be seen that the control module 12 alsohouses an air compressor 116 used to provide compressed air for thepneumatic circuit in the APM. Compressed air from the compressor 116 ispartially refrigerated in an after-cooler 118 and any moisture in theair is then removed as it passes through a water trap 120 and dryer 122.It is important that the air stream used to fluidise the plasticsmaterial powder is moisture free so that no moisture enters the mould 40with the powder, which would have a deleterious effect on the quality ofthe rotamoulded product. The cooled air from the dryer 122 then passesthrough an air filter 124 and a pressure regulator 126 before passinginto the remainder of the pneumatic circuit in the APM 10. A pressureswitch 128 continually monitors the air pressure from regulator 126 toensure that it is maintained within a prescribed range. If the airpressure falls below a predetermined minimum threshold or exceeds apredetermined maximum threshold, a signal is transmitted to the computerterminal 24 via a remote terminal unit (RTU) 130. The RTU 130 in controlmodule 12 comprises a programmable microprocessor and other integratedcircuits to provide analog to digital conversion of signals from varioussensors in the APM, for example, the pressure signal from pressureswitch 128. The RTU 130 also provides serial to parallel processing andvice versa of control signals to and from the computer terminal 24 via aradio frequency link 131 and a modem 132. The RTU 130 constantly updatesthe various digital end analog inputs from the APM 10, and once everysecond the computer terminal 24 polls each of the RTUs to obtain currentvalues held in memory registers in the RTUs. Each of the pneumaticcylinders employed in the APM 10 such as, for example, the cylinder 134for actuating the diverter valve 36, are provided with proximity sensorsto detect when the pneumatic ram is in its fully opened and fully closedpositions.

A three phase electrical power supply is connected to the compressor116, after-cooler 118, dryer 122, the electrical motors driving meteringmeans 28 and the electric motor driving the conveyor 20, via overloadcircuit breakers 136 and relays 138. Each of the relays 138 is connectedto the RTU 130 so that the supply of electrical power to each part ofthe control module is under direct control of the computer terminal 24.Also illustrated in FIG. 8 are four pneumatic valves 140 for controllingthe supply of compressed air to the fluidiser tubes 32, exhaust valve 35and the pneumatic ram 134 for diverter valve 36.

The arrangement of each rotatable mould and its associated controldevices in the respective production modules 14 will now be describedwith reference to FIGS. 2, 4, 5, 9(a) and 9(b).

In FIG. 2, a rotatable mould 40 is visible in only one of the productionmodules 14, although the other two production modules 14 also house arotatable mould assembly 42 which is substantially identical to thatillustrated. FIGS. 4 and 5 illustrate in greater detail one of therotatable mould assemblies 42 employed in the APM 10.

The rotamoulding mould 40 is mounted for biaxial rotation about firstand second perpendicular axes 44 and 46 respectively. The mould 40 isfixed to a first support frame 48 which is rotatably mounted forrotation about the first axis 44, and which is in turn supported by asecond support frame 50 rotatably mounted for rotation about the secondaxis 46. Each of the first and second support frames 48, 50 are drivenby a respective electric motor (with gear box) 52 via a chain andsprocket transmission 54. The motor and transmission for driving thefirst support frame 48 are mounted on the second support frame 50,whereas the motor and transmission for driving the second support frame50 are mounted on a structural frame 56 of the APM 10. Electric motor 52for driving the second support frame 50 is provided with a variablespeed drive (VSD) unit preset to ramp the speed of the motor up and downat a predetermined rate whenever the motor 52 is switched ON or OFFrespectively.

A first rotary joint 58 is adapted to allow the air stream carrying theplastics material powder to pass from one side of the joint on theoutside of support frame 50, which remains stationary relative to therotating mould 40, to the other side of the joint which rotates with thefirst support frame 48 about the first axis 48. A second rotary joint 60is mounted on a structural frame 56 and is connected to the first rotaryjoint 58 via a transfer line extension 62. The first rotary joint 58orbits about the second axis 46. The second rotary joint 60 is adaptedto allow the air stream from line extension 62 to pass from one side ofthe joint 60 on the outside of structural frame 56, which remainsstationary relative to the rotating mould 40, to the inside of thesecond support frame 50 which rotates with the mould 40 about the secondaxis 46. The first and second rotary joints 58, 60 operate to allow theplastics material powder carried in the air stream via transfer line 38to be delivered to the mould 40 while it is biaxially rotating about thefirst and second perpendicular axes 44, 46.

The specific construction and operation of the rotary joints 58, 60 isdescribed in detail in the specification which accompanied commonlyowned Australian Patent Application No. AU 42099/96 filed on 19 Jan.1995, and will not be described in detail again here.

As can be seen most clearly in FIG. 5, each rotatable mould assembly 42of the APM 10 incorporates a pair of pallet moulds 40 comprising firstand second components 64 and 66 forming the lid and base respectively ofthe respective moulds. In FIG. 5 the top mould 40 is shown in an openedposition, whereas the bottom mould 40 is shown in a closed position. Thelid 64 and base 66 of each pallet mould are shaped to produce thedesired configuration of the finished rotamoulded pallet, and include aseries of nine depressions 68 in the base 66 of the mould to form thefeet of the finished pallet. In this embodiment, the lid 64 of thepallet mould is moveable relative to the base 66 between a firstposition in which the mould is closed and ready to receive a shot ofplastics material powder and a second position in which the mould isopened and the finished rotamoulded pallet can be removed. Each mould 40is also provided with actuating means in the form of a pair of pneumaticrams 70 for moving the lid 66 of the mould between the opened and closedpositions.

An important feature of the present invention is that each rotatablemould 40 is provided with heating means in connection therewith forheating the mould directly during rotation. In this embodiment, theheating means comprises a plurality of electric heating elements 74mounted in close proximity to predetermined areas of the first andsecond components 64, 66 of the moulds 40. The arrangement of theheating elements 74 can be seen most clearly in FIG. 4. The heatingelements 74 are mounted in thermal contact with the respective externalsurfaces of the first and second components 64, 66 and are covered by aninsulating jacket 76 designed to ensure that most of the heat generatedby the heating elements 74 is reflected and/or conducted into the moulds40. The first and second components 64, 66 of the moulds 40 aretypically manufactured from a material having a high thermalconductivity, such as cast aluminium, and the insulating jacket 76comprises a glass fibre insulating mat with a reflective aluminiumvacuum-formed skin and housed within a steel casing 78. The electricheating elements 74 are capable of heating the moulds 40 to temperaturesexceeding 200° C., however even at maximum heating temperatures, theexternal surface of the steel cladding 78 only remains warm to thetouch.

Electrical power for the heating elements 74 is transmitted to therotating moulds 40 via first and second electrical rotary contacts 80,82 provided on the first and second axes of rotation 44, 46, in asimilar manner to that of the first and second rotary joints 58, 60. Inthis embodiment, the heating elements 74 are arranged on the first andsecond components 64, 66 of the mould in such a way as to providesubstantially uniform heating over the entire internal surface area ofthe mould. However, clearly the heating elements 74 could be arranged inconnection with the mould so as to provide any desired thermal profileover the inner surface of the mould. Furthermore, each of the heatingelements can be operated independently so as to increase or decrease theheat applied to selected areas of the mould. This may be advantageous,for example, where additional thickness is required in the wall of therotamoulded product in selected areas. Additional heat could be appliedin the selected areas so as to accelerate the deposit of melted plasticsmaterial powder in those areas relative to the remainder of the innersurface of the mould.

Each rotatable mould 40 is also provided with sensing means inconnection therewith for sensing the temperature of the mould andgenerating a temperature signal which is transmitted to the electricalcontrol means via the electrical rotary contacts 80, 82. The sensingmeans typically comprise a plurality of temperature sensing devices,such as, for example, K-type thermocouples 84, embedded at predeterminedlocations in at least one of the first and second components 64, 66 ofthe mould. Typically, both the lid 64 and base 66 of the mould areprovided with two thermocouples 84, at least one of which is arranged tosense the temperature at the inner surface of the mould. The otherthermocouple 84 may be arranged to sense the temperature at the externalsurface of the first and second components 64, 66 respectively of themould. The two halves of a mould may have different temperature profilesdepending on the thickness of the melted plastics material.

The rotatable mould assembly 42 also incorporates means for injectingthe plastics material powder into the mould 40. Typically, the means forinjecting the powder comprises an injector 90, (see FIG. 5) having aninlet 92 and an outlet 94 adapted to extend into the mould 40 forintroducing the plastics material powder into the mould. The air streamcarrying the fluidised plastics material powder is delivered to theinjector 90 via a second transfer line extension 98 from the firstrotary joint 58. A pneumatic ram 99 is provided for moving the injector90 between a first position in which the first outlet 94 extends intothe mould (as illustrated in FIG. 5), to allow the plastics materialpowder to pass into the mould, and a second position in which the firstoutlet 94 is withdrawn to prevent any further plastics material powderfrom passing into the mould. Only one of the moulds 40 illustrated inFIG. 5 is shown fitted with an injector 90, however both moulds 40 aretypically provided with an injector for delivering powder to the mouldduring rotation. The structure and operation of injector 90 aredescribed in detail in the specification which accompanied AustralianPatent Application No. AU 42099/96 and will not be described again here.

The first and second components 64, 66 of the pallet mould have coolingmeans provided in connection therewith to enable forced cooling of themould 40 to accelerate solidifying of the rotamoulded product at the endof the heating cycle. The cooling means in this embodiment comprises apassage, (not visible in the drawings) extending through the casing 78adjacent to the outer surfaces of the lid 64 and base 66 respectively ofthe moulds 40. Air movers 101 are connected to inlets 100 to the airpassages for pumping cooling air through the passages and out throughthe cooling air exits 102. The cooling air exits 102 are arranged remotefrom, and with their axes substantially perpendicular to, the air inlets100 so that the cooling air is forced to circulate within the casings78. A proportion of the air pumped through the air passage may berefrigerated air in order to enhance the cooling effect.

FIG. 9(a) illustrates schematically part of the pneumatic circuit, flowpath of powder and the electric power and control circuit external tothe rotatable mould assembly 42 of a production module 14. An RTU 144performs similar functions to the RTU 130 in the control module 12. Apair of overload circuit breakers 136 and relays 138 provide three-phasepower to the electric motors 52 on the rotatable mould assembly 42.Electrical power is supplied directly to the motor 52 mounted on thestationary structural frame 56, and is supplied via the rotary contact82 to the motor mounted on the second rotatable support frame 50.Three-phase power is also supplied to the electrical heaters on themoulds 40 via rotary contact 80. RTU 144 also controls a pneumaticcylinder 148 via pneumatic valve 150, for controlling the supply offluidised plastics material powder to the rotatable mould assembly 42via diverter valve 152. The powder is delivered to the rotating mouldvia rotary contacts 60 and 58 as previously described. The pneumaticcylinder 112 used for actuating the lifting arm 110 of the palletstacker is also controlled by RTU 144 via pneumatic valve 154. Eachproduction module 14 is provided with two RTUs, the first RTU 144 beinglocated external to the rotatable mould assembly 42, and a second RTU160 (see FIG. 9(b)) being provided internal to the rotatable mouldassembly 42.

FIG. 9(b) is a schematic illustration of the pneumatic circuit, the flowpath of powder, and the electric power and control circuits locatedwithin the first support frame 48 of the rotatable mould assembly 42 inone of the production modules. Both the pneumatic and electric circuitsare under the control of RTU 160 which perform similar functions to theRTU 130 in the control module 12. The supply of three-phase electricalpower to each of the heating elements 74 provided in connection with thefirst and second components 64, 66 of the moulds 40, is controlled byRTU 160 via circuit breakers 136 and relays 138. RTU 160 also controlsthe operation of each of eleven pneumatic cylinders via pneumatic valves162. Two of the valves 162 control the respective pairs of pneumaticcylinders 70 for moving the lids 64 of the pallet moulds between theopened and closed positions. Each of the pneumatic rams 70 are locked inthe opened or closed position by a lock mechanism 164 which is releasedby a pneumatic cylinder 166. Hence, should the supply of compressed airfail for any reason, the moulds 40 will remain locked in their opened orclosed position. Two of the pneumatic valves 162 control each pair oflock pneumatic cylinders 166. Two more pneumatic valves 162 control thesupply of compressed air to the cylinders 99 for moving the cyclones 90into and out of the moulds 40. Another one of the pneumatic valves 162controls the supply of compressed air to a pneumatic cylinder 168 whichcontrols a diverter valve 170 that controls the supply of fluidisedplastics material powder to the respective moulds 40 via cyclones 90. Atany one time, fluidised powder is supplied to only one of the moulds 40.Finally, each of the air movers 101 supplying cooling air to the casing78 for each mould is controlled by a separate pneumatic valve 162.

RTU 160 also receives numerous inputs from various sensing devices. Eachof the pneumatic rams 70, 99 and 168 are fitted with proximity sensorsso that the computer terminal 24 can detect the position of the firstand second components 64, 66 of each mould, the position of the cyclones90 and the position of the diverter valve 170 at any time, by pollingRTU 160. RTU 160 also receives inputs from each of the thermocouples 84so that the computer terminal 24 can monitor temperature variations bothat the inner surfaces of the lid 64 and base 66 of each mould as well aswithin the casing 78. The number of inputs and outputs to the RTU 160may be increased to accommodate additional sensing devices and controlfunctions if required.

FIGS. 6 and 7 illustrate a preferred form of pallet stacker 22 forstacking the pallets 104 as they exit from the APM 10. As shown in FIG.6, when a pallet 104 is ejected from the rotatable mould assembly 42 itis carried by the conveyor 20 to the pallet stacker 22. The palletstacker 22 operates by stacking the pallets from the bottom up. As theconveyor 20 conveys a pallet into the bottom of the stacker the pallet104 eventually strikes a stop at an end wall 106 of the stacker. Thisautomatically triggers a lifting mechanism within the stacker whichoperates to lift the pallet 104 a prescribed height where it is held bya plurality of toggle latches 108 to make room for the next palletunderneath. The lifting mechanism comprises a pair of lifting arms 110actuated by a respective pneumatic ram 112 which is speed-adjustable andself-returning. When the pneumatic rams 112 are activated the liftingarms 110 lift the latest pallet 104, (together with any earlier palletsalready in the stacker) to a height just above the toggle latches 108.Toggles latches 108 pivot inwards to allow the latest pallet 104 to moveupwards, and then pivot outwards again to support the pallets in astacked position when the lifting arms 110 return to their loweredposition. The stacker 22 is then ready to receive the next pallet 104.

If desired, means for marking each of the finished pallets may beprovided adjacent to or in connection with the pallet stacker 22. Suchmarking means may comprise, for example, an ink jet printer for printingthe manufacturer's product name, trade mark and/or serial number on eachpallet.

The control sequence for the APM 10, as effected by a process controlprogram provided in one of the computer terminals 24, will now bedescribed in detail. Referring to FIG. 11, the first step 200 after thepower is switched ON requires the machine operator to log on to theterminals 24. Then at step 202 the desired recipe for the rotamouldedproduct is loaded into the computer terminal, for example, from a floppydisc. A typical process control recipe for a rotamoulded product mayspecify the following control parameters:

(i) The "cooking" temperature-time characteristic

(ii) The quantity (weight) of powder in each shot

(iii) The number of dumps (shots) of powder

(iv) The sequence and timing of dumps relative to the temperature-timecharacteristic

(v) The start and stop positions of the rotatable mould assembly

(vi) The ratio of rotation

(vii) The mould finish temperature

The ratio of rotation R_(rot) may be calculated as follows: ##EQU1##Where V_(min) =the speed of rotation of the mould about the minor axisof rotation (in RPM).

V_(maj) =speed of rotation of the mould about the major axis of rotation(in RPM).

The ratio of rotation R_(rot) may vary depending on the shape of theproduct, in order to achieve the desired distribution of plasticsmaterial powder over the inner surface of the mould.

FIG. 11 illustrates a typical temperature-time characteristic employedin the manufacture of a rotamoulded plastic pallet having a high densitypolyethylene (HDPE) skin filled with foamed polyethylene. The powderused to form the HDPE skin is the first shot or dump in the rotamouldingprocess, and the foamed polyethylene interior is the second shot or dumpin the process.

The vertical axis in the graph of FIG. 11 illustrates the temperature atthe inner surface of the mould where the powder first starts to melt.The initial temperature of the mould at time T₀ is above ambient and isthe temperature to which the mould is cooled (in a previous cycle) toallow the pallet to solidify sufficiently to be ejected from the mould.

Once the recipe has been loaded at step 202 in FIG. 10 the processcontrol program in computer terminal 24 sets and confirms all variablesprovided from the recipe at step 204. At step 206 the program checks thepowder, air and power circuits in the APM and continues to monitor alloverload and alarm states. The APM is now ready to commence automaticmass production of plastic pallets. At step 208 the rotamoulding processcommences in accordance with the recipe when instructed to start by themachine operator. This is represented as time T₀ in FIG. 11. At time T₀the process control program switches ON the electrical heating elementson the two moulds in one of the rotatable mould assemblies 42 at step210, and simultaneously meters the prescribed quantity of plasticsmaterial powder for the first shot at step 212 according to the recipe.At step 214, when the moulds reach a prescribed temperature set by therecipe and sensed by the thermocouples 84, the first shot of powder isdumped to the mould at step 216. Although not indicated in FIG. 10,rotation of the rotatable mould assembly 42 also commences at step 210when heating of the moulds commences, or sometime thereafter but priorto step 216 when the first shot of powder is dumped to the moulds.

At step 218 the control program commences the "cooking" cycle inaccordance with the temperature-time characteristic provided in therecipe. The temperature of the moulds and the melt is continuallymonitored and electrical power to the heating elements 74 is modulatedin order to track the prescribed temperature-time characteristic asclosely as possible. At time T₁ the dump starts to melt and there is anoticeable change in the temperature characteristic due to theendothermic reaction.

Meanwhile, the control program meters a second shot of plastics materialpowder at step 212 into one of the primary hoppers 26. The second shotwill form the foamed plastic inner matrix of the plastic pallets. Whenthe first dump is cooked at step 220 the second shot of powder is dumpedat time T₂ represented by step 222 in FIG. 10. Meanwhile, the controlprogram proceeds to meter another shot of powder at step 212, which willbe the first shot for the next pallet. At time T₃ the second dump ofpowder starts to melt and the temperature of the moulds is maintained inaccordance with the recipe as represented at step 224 in FIG. 10. Attime T₄ the electrical heating elements are switched OFF and the coolingcycle commences at step 226. The moulds are allowed to cool naturallyfor a prescribed time in order to allow crystallisation and/orsolidification of the melt to occur. At time T₅ forced cooling of themoulds is commenced by turning on the air movers 101. At time T₆ thecooling cycle is completed and at step 228 the mould is set for openingand ejection of the pallet. During the period between times T₆ and T₇the rotatable mould assembly 42 is rotated to a first position in whichthe respective moulds are opened and further cooling of the palletoccurs, and then it is rotated to a second position in which the palletcan fall out of the mould under its own weight onto the conveyor 20 asillustrated in FIG. 6. In order to enable the control program tocorrectly position the mould assembly, a proximity switch on the outersupport frame 50 is activated when the VSD unit on motor 52 is actuatedduring deceleration. An optical sensor on the inner frame 48 enables theAPM to stop the inner frame in the desired position. The control programis to some extent self-correcting so that if, for example, a pallet doesnot drop out under its own weight the rotatable mould assembly 42 isrotated through a predefined sequence before trying again. Ejection ofthe pallet is indicated at step 230 in FIG. 10. When both pallets havebeen ejected from the rotatable mould assembly 42 the mould is closed atstep 232. The control program returns to the beginning of the heatingcycle at step 210, and the heating cycle recommences at time T₈ ifanother pair of pallets is to be produced.

The heating cycle for each of the production modules 14 of the APM 10are operated sequentially so that at any one time three-phase electricalpower is being supplied to the electrical heating elements of only onerotatable mould assembly 42 in order to minimise power consumption.Obviously, if overload of the power supply is not an issue some overlapin the heating cycles can be tolerated. Also, although in FIG. 10 thereis reference to only one mould, both moulds in each production moduleoperate substantially simultaneously. The only exceptions are during thedelivery of powder to the respective moulds at step 216, which isperformed sequentially by operating diverter valve 170 (see FIG. 9(b)),and at step 230 when the pallet is ejected from one mould and then theother. With the sequential operation of the production modules describedabove, the APM 10 produces a new pair of plastic pallets approximatelyevery ten minutes.

The APM 10 has a number of significant advantages over conventionalmethods of producing plastic pallets, not least of which is thesubstantial reduction in the cost of producing the pallets. Advantagesof the APM include, but are not limited to:

(a) Labour content in production is all but eliminated

(b) Decreased production cycle time due to direct heating of the moulds

(c) Automation of the double dump procedure made possible by powderdelivery means

(d) Automatic ejection, conveying and stacking of the finished pallets

(e) Improved quality control.

Now that a preferred embodiment of the automatic rotamoulding processand method of control have been described in detail, it will be apparentto persons skilled in the relevant mechanical, electrical androtamoulding arts that numerous variations and modifications may bemade, in addition to those already described, without departing from thebasic inventive concepts. For example, the rotatable mould may have morethat two components if more complex shapes are involved. Furthermore,the number of modules and their arrangement can be varied, and eachproduction module may be set up to produce different products. In theAPM three production modules were selected due to the typical productioncycle time for the rotamoulded pallets. Furthermore, each productionmodule 14 can operate independently of the other modules under thecontrol of a remote computer if desired. In addition, any suitabledelivery means for delivering plastics material powder to the mouldsduring rotation can be employed, although the delivery means describedin Australian Patent Application No. PN0666 is preferred. Other forms ofheating means for directly heating the moulds can also be employed, forexample, a fan forced gas fired heat exchanger. Hot air is pumpedthrough passages provided in or immediately adjacent to the moulds toheat them to the desired temperature. This arrangement has the advantagethat the same passages can also be used to pump a coolant through tocool the moulds. The thermal profile over the inner surface of themoulds can be designed by careful selection of the configuration of thepassages. The coolant may include water or water vapour, and may also bedesigned to induce an evaporative cooling effect. All such variationsand modifications are to be considered within the scope of the presentinvention, the nature of which is to be determined from the foregoingdescription and the appended claims.

The claims defining the invention are as follows:
 1. An automatic rotamoulding apparatus comprising:a rotatable mould mounted for rotation, having heating means provided in connection therewith for heating the mould directly during rotation and sensing means provided in connection therewith for sensing the temperature of the mould; delivery means for automatically delivering a plastics material powder to the mould during rotation; and, electronic control means operatively connected to said delivery means, said heating means and said sensing means, for controlling the delivery of plastics material to the mould and for controlling the temperature of the mould throughout the rotamoulding process responsive to said sensing means whereby, in use, a rotamoulded product can be produced in the mould without manual intervention.
 2. An automatic rotamoulding apparatus as defined in claim 1, wherein said rotatable mould is one of a plurality of rotatable moulds mounted in a plurality of modules, each module being independently operable under the control of said electronic control means.
 3. An automatic rotamoulding apparatus as defined in claim 2, wherein each mould comprises first and second components movably mounted within a first support frame adapted for rotation about a first axis, actuating means being provided for automatically moving said first component of the mould relative to the second component between a first position in which the mould is closed and ready to receive a shot of plastics material powder, and a second position in which the mould is opened and the finished rotamoulded product can be removed.
 4. An automatic rotamoulding apparatus as defined in claim 3, wherein said actuating means comprises a pneumatic ram operatively connected to said electronic control means for moving the first and second components between said first and second positions.
 5. An automatic rotamoulding apparatus as defined in claim 4, wherein said heating means comprises a plurality of electric heating elements provided in thermal contact with the first and second components of the mould and said sensing means comprises a plurality of temperature sensing devices provided in connection with the first and second halves of the mould in predetermined locations.
 6. An automatic rotamoulding apparatus as defined in claim 2, wherein each rotatable mould further comprises cooling means to facilitate forced cooling of the mould following heating.
 7. An automatic rotamoulding apparatus as defined in claim 6, wherein said cooling means comprises a passage extending proximate to said first and second components of the mould and pumping means operatively connected to said electronic control means for pumping cooling medium through said passages.
 8. An automatic rotamoulding apparatus as defined in claim 1, further comprising conveyor means for conveying a finished rotamoulded product from the mould to a location remote from the mould.
 9. An automatic rotamoulding apparatus as defined in claim 8, wherein the apparatus is also provided with stacking means for stacking the finished rotamoulded products as they arrive at said remote location.
 10. A method for controlling an automatic rotamoulding process using an electronic control means operatively connected to a delivery means, a heating means and a sensing means, the method comprising the steps of:metering a first prescribed quantity of plastics material powder into a hopper to be delivered to a rotatable mould as a first shot; operating said heating means provided in connection with the mould to heat the mould; actuating said delivering means, in response to said sensing means sensing a prescribed temperature of the mould, to deliver the first shot of powder from said hopper to the mould and simultaneously rotating the mould so as to distribute the powder over an interior surface of the mould; monitoring said sensing means provided in connection with the mould to sense the temperature of the mould during rotation; controlling the operation of said heating means in accordance with a predetermined temperature-time characteristic and responsive to said monitoring of the sensing means; cooling the mould; and, ejecting the finished rotamoulded product from the mould.
 11. A method for controlling an automatic rotamoulding process as defined in claim 10, further comprising the steps of:metering a second prescribed quantity of plastics material powder into a hopper, to be delivered to the rotatable mould as a second shot; and, actuating the delivery means to deliver the second shot of powder from the hopper to the rotating mould.
 12. A method for controlling an automatic rotamoulding process as defined in claim 10, further comprising the steps of:conveying the finished rotamoulded product on a conveyor to a stacking apparatus; and, stacking the finished rotamoulded product to form a stack of such finished rotamoulded products. 